Method and apparatus for determining the rate of flow of particles in a vacuum deposition device

ABSTRACT

A method of determining the rate of flow of particles in a vacuum deposition device, using the relationship that the rate of flow of the particles is equal to the density of the particle current times the mean velocity of the particles, comprises establishing a flow or current of particles in the device and directing a laser beam through the current of particles in order to measure the atteunation of the beam after it has passed through the particles and thus to determine the vapor density within the current. The absorption wavelength shift of the laser beam provides an indication of the velocity component of the current because the shift is produced by a Doppler effect by the velocity component of the particle current in the direction of flow from the source of the particles to the substrate to be coated. In a preferred embodiment of the method, a housing has a vacuum chamber in which the substrate to be coated is held in a position across the chamber from a source of the coating material which is evaporated or sputtered so that a current of particles moves from the source across the chamber to the substrate. At least one laser beam is directed through the particles and is picked up on the opposite side by a detector which is capable of measuring the absorption wavelength shift. Two laser beams may be directed through the particle stream, including one transverse to the current direction and one parallel to the current direction. The second beam may be provided by using a semi-transparent mirror to deflect the original laser beam so that it is passed through the chamber again in a direction at substantially right angles to the first beam direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to a new useful method and fordetermining the rate of flow of particles in a deposition device duringthe coating of a substrate.

2. Description of the Prior Art

By rate of particle flow is meant the quantity of particles flowingwithin one second through a predetermined cross-sectional area of theparticle current. In the deposition of thin layers on substrates, themeasuring of the rate of particle flow is of particular importance sincethe properties of the layer (structure, constitution, variation of thematerial composition of the layer due to chemical reactions of theparticles during the deposition, etc.) are appreciably influenced by thevelocity of deposition so that, for a reproducible application of thinlayers, a continuously uniform rate of the particle flow is necessary.Usually, the particle currents needed for the application of thin layersare produced by thermic evaporation or by atomization of the material tobe deposited, effected by ion bombardment (for example, by cathodesputtering in rare gases). The substrates to be coated are positioned inthe particle current. The rate of condensation on the substrate dependsnot only on the rate of particle flow but, in addition, on thecoefficient of condensation.

The devices for deposition plants, up to date known as ratemeters, arebased on the measurement of the vapor density ρ in the evaporationcurrent in the space between the vapor source and the substrates.Usually, this density is determined by measuring the vapor pressure bymeans of an ionization manometer into which the vapor particles aredirected. Its relation to the evaporation rate R is given by the formula

    R = ρ . v

where v is the average velocity of the vapor particles in the directionfrom the source to the substrate. This velocity has not been measured invacuum-deposition devices as yet. The use of a vapor density measuringinstrument as a ratemeter was based on the assumption that, in a certainevaporation process in a given device, a measured vapor density isalways associated with a definite velocity v which remains constant frommeasurement to measurement. This assumption, however, in general, is notcorrect with a sufficient accuracy so that a secure measuring of therate merely by measuring the vapor density is not possible.

In addition, in the known measuring instruments, which must be locatedwithin the deposition device, disturbances occur frequently insofar asparts of the measuring equipment, for example, the electrodes of anionization manometer, become coated themselves.

Another kind of known ratemeter uses the variation of the naturalfrequency of an oscillator quartz located in the device, which frequencyvariation results from the coating and is determined by means ofelectronic measuring equipment. From the frequency variation, the massdeposited per time unit on the quartz surface and, therefrom, the numberof particles passing to condensation may be determined. These deviceshave the disadvantage that they make possible the measuring of the rateof flow only during a limited period of time since the layer depositedon the quartz must not exceed a certain thickness.

Further drawbacks of the known oscillator-quartz ratemeters are thedisturbances which are caused by the heat radiation of the evaporationsource and by the condensation heat which is released on the quartzsurface during the coating thereof. In addition,oscillator-quartz-measuring equipments are susceptible to troublescaused by electric gas discharges which occur in deposition devices,primarily in the various processes of the atomization of solid matter byion bombardment.

SUMMARY OF THE INVENTION

The present invention is directed to a method permitting a directdetermination of the rate of particle flow in a vacuum-deposition deviceso that, with the knowledge of the coefficient of condensation, the rateof condensation can be exactly measured and controlled.

In accordance with the invention, the particle current is traversed by alaser beam and, at the same time, the attenuation of the intensity ofthe beam due to absorption is measured, wherefrom the density of theparticle current is determined, which simultaneously makes it possibleto measure the absorption-wavelength shift. The shift is produced,because of a Doppler effect, by the velocity component of the particlecurrent in the direction from the source to the substrate. The density ρof the particle current and its velocity v as measured by the laser beamshift are then used for determining the rate of particle flow as theproduct ρ. v.

Accordingly it is an object of the invention to provide an improvedmethod of determining the rate of flow of particles in a vacuumdeposition device, using the relationship that the rate of flow of theparticles is equal to the density of the particle current times the meanvelocity of the particles which comprises establishing a current ofparticles in the device and directing a laser beam through the current,and thereafter detecting the laser beam absorption wavelength shift byobserving the beam after it has passed through the current and using thevalue of the shift as an indication of the velocity component of thecurrent which is produced because of a Doppler effect as the laser beamis passing through the current.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this disclosure. For a better understanding of the invention,its operating advantages and specific objects attained by its uses,reference should be had to the accompanying drawing and descriptivematter in which there are illustrated preferred embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWING

The only FIGURE of the drawing is a schematic representation of a vacuumdeposition device having means for determining the rate of flow of theparticles constructed in accordance with the invention.

GENERAL DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing in particular, the invention method isperformed in a vacuum deposition device which is provided with means fordetermining the rate of flow of particles from a source which is acoating material, which move in a stream or current across the vacuumchamber and are deposited in thin layers on a substrate which is held inthe chamber. The vacuum deposition device includes a casing C which hasan interior vacuum chamber V. Within the vacuum chamber there are twosouces Q_(A) and Q_(B) which produce a current of particles and permit adeposition upon substrates Su of two different substances in successionor simultaneously. The sources designated Q_(A) and Q_(B) include themeans for holding the material to form the coating as well as the meansfor effecting the thermic evaporation or atomization of the material tobe deposited by ion bombardment such as by cathode sputtering. Thesubstrates Su are secured to a rotary carrier plate P which is rotatedby a motor M through a transmission T. For accurately determining thedeposition times pivotal cover masks B_(A) and B_(B) are providedadjacent the sources Q_(A) and Q_(B). A heating element H makes itpossible to keep the substrates at a temperature which is optimal forthe respective coating processes during the deposition.

In accordance with the invention, the device includes at least one laserradiation source which, in the present example, comprises an organic dyelaser FL which is pumped by a nitrogen laser N. Organic dye lasers havethe advantage that they can be continuously tuned within a definiterange of wavelengths. In the embodiment shown, a control device EM actson a grating G to effect the tuning as desired.

The beam produced by the organic dye laser FL is directed into thevacuum deposition device through a window F, passes through the currentof particles produced by the sources Q_(A) and/or Q_(B) for evaporationor cathode sputtering, and leaves the vacuum chamber through anoppositely located window F'. Thereupon the beam falls on a detector D₁which makes it possible to measure the optical transmission T₁ of theparticle current at the respective laser wavelengths.

In order to protect the windows from being coated by stray particles,the apparatus includes a plurality of apertured screens A which areassociated with each window F, F'. These screens A permit an unhinderedpassage of the laser beam but prevent the stray particles from passingto the window.

For carrying out the invention a second laser beam is particularlyadvantageous and in some instances necessary so that a traverse of theparticle current in a second direction preferably parallel to the maindirection of the current may be effected. In order to save the expenseof a second laser a partial beam may be split off the first laser beamby means of a semi-transparent mirror Sp. The split beam is deflected byfurther mirrors R so as to direct the second beam substantially parallelin the direction of particle flow. The second beam moves through awindow F" and through the particle stream to an opposite window F'".Screens A are also associated with these windows to protect the windowsagainst being coated by the material which is evaporated. The beampassing through the window F'" enters a second detector D₂ whichindicates the transmission T₂ of the particle current in the directionparallel to the main current direction.

By transmission of the particle current is understood the ratio of theintensity I of the laser beam after passage through the particle currentto the intensity I₀ of the laser beam prior to entering the particlecurrent. I₀ is either known in a laser which has a sufficient stabilityor it can be measured when there is no particle flow by one of thedetectors which are employed. Another advantageous possibility ofdetermining the intensity of the laser beam before it passes through theparticles is to measure the intensity of the transmitted radiationalternately at two closely adjacent wavelengths of which only one isabsorbed, and therefrom to compute the ratio of the absorbing wavelengthI to the non-absorbing wavelength I₀.

As to the theoretic basis of the inventive method, it is true that, forthe given laser wavelength, the particle current has differentcoefficients of optical absorption in the two directions. Thisphenomenon is due to the unequal magnitude of the Doppler effect, causedby the particle motion, in the two directions of measurement. Dependingon the direction of motion, the absorption lines of the particles (forexample, of a vapor in a vacuum evaporation device) appear shiftedrelative to the position of these lines for particles at rest. In thesimplest case, the absorption is measured at a right angle to thedirection of motion of the particles without a Doppler effect (i.e. theparticles have no notable velocity component in the direction of themeasuring light beam); while, with the second beam of measuring light,parallel to the direction of motion of the particles, a maximum Dopplereffect appears.

If the transmission of the particle current is measured in twodirections at a laser wavelength for which the particles are absorbing,the rate of particle flow can be determined from the formula

    R = c.sub.1 (log T.sub.1) c.sub.2 (T.sub.2 - T.sub.1.sup.c3 )

where T₁ and T₂ are the transmissions of the particle current for thelaser beam in the two mentioned directions, and c₁, c₂ and c₃ areconstants which depend on the geometric arrangement and on theabsorption coefficient of the particles to be measured. The constantsmay be determined in advance by test measurements.

The described measurement can be effected not only at the point of themaximum of an absorption line but also at a slightly differentwavelength, at the edge of the absorption line, whereby two advantagesare obtained. The sensitivity is increased since, because of the greatersteepness of the edge, the velocity of the absorption variation with theshifting of the position of the absorption line due to the Dopplereffect is higher than at or close to the maximum absorption. Further,the measuring at the edge has the advantage that the variation of themeasuring signal is approximately proportional to the velocity of theparticle current.

In a test case, titanium metal has been thermally evaporated in thevacuum chamber in order to produce a layer of pure titanium. Thestarting titanium metal may contain impurities, for example, TiO. TiOmight also be formed by a chemical reaction with the crucible or withthe residual-gas atmosphere. Consequently, the particle current formedalmost exclusively of titanium atoms contains a certain proportion ofTiO molecules. The inventive method makes it possible to measureseparately the two rates of particle flows during the evaporation. Formeasuring the rate of flow of the titanium particles, a radiation havingthe wavelength λ = 629.7 nm has been used and the correspondingtransmission T₁ = 0.643 in the direction perpendicular to the mainmotion direction of the vapor current determined. This means that, atthe mentioned wavelength and under the given geometric conditions, thetitanium vapor current had an absorption of 35.7%. For the transmissionparallel to the direction of the vapor current, the value T₂ = 0.837 hasbeen found. For the used experimental arrangement, the constants havebeen determined in advance as c.sub. 1 = - 1.95×10¹² cm⁻³, c₂ = 1.64×10⁵cm sec⁻¹, and c₃ = 1.35. After introducing the measured transmissions T₁and T₂ into the above mentioned formula, the resulting rate of particlecurrent was 1.75×10¹⁶ particles per cm² and sec.

For the analogous measuring of the TiO particle flow, the wavelength λ =615.9 nm has been used at which TiO particle flow possesses oneabsorption band. In this case, the results were T₁ = 0.942, T₂ = 0.968and, therefrom, the rate of the TiO particle flow was calculated as2.13×10¹³ of TiO molecules per cm² and sec.

With the evaporation of nickel, the rate of flow of the Ni particlescould be determined in the same manner through absorption at 388.2 nmand the rate of flow of NiO molecules occurring at the same time hasbeen determined by means of the wavelength λ = 517.5 nm.

In a similar way, the rate of flow of all particles present in theparticle current can be determined separately, provided a suitableabsorption line is available. This applies not only to neutral atoms andmolecules but also to ions for which the absorption spectrum issubstantially different as compared to that for the neutral particles.Thus, the invention offers the possibility of a complete control of theintensity and composition of the particle current in a vacuum depositiondevice. Particularly advantageous is the application of the inventivemethod in the evaporation of mixtures, for example, alloys, where itpermits following of the mixture ratio of the individual components andusing of the measured values for controlling the evaporation process.

In another test, first T₁ is measured at a preselected point of thespectrum corresponding to absorption maximum of the kind of particles tobe traced and, thereupon, the least wavelength is changed by Δ λ so asto obtain a minimum for T₂. This leads to a maximum of absorption in theT₂ direction for the changed wavelength. The rate of particle flow thenresults from the formula

    R = c.sub.1 (log T.sub.1) c.sub.4 Δ λ

where c₄ is a constant depending on the geometric arrangement and thecoefficient of absorption of the particles to be measured.

For example, iron has been evaporated. While measuring in the directiontransverse to the vapor current (so that no Doppler effect occurs), ironhas an absorption line (position of the maximum) at 385.990 nm. In thedirection of motion of the vapor current, on the contrary, a greatertransmission is obtained with the same wavelength, since due to theDoppler effect, the absorption line appears shifted toward longerwavelengths. In the present example, a minimum of transmission has beenobtained upon increasing the wavelength by Δ λ = 1.2×10⁻³ nm, i.e. uponadjusting the laser light to 385.9912 nm. This means, that in the motiondirection, the iron vapor had a maximum of absorption at this wavelength. An introduction of the measured values T₁ and Δ λ into the lastmentioned formula results in a value of 1.73×10¹⁶ for the rate of flowof the Fe particles. Here again, c₄ has been determined in advance by atest measurement.

The above mentioned formulas have been established in accordance withthe well known theory of the Doppler effect and the measurement of vapordensities based on their optical absorption. The theory of these twophenomena is known per se and there is no need for an explanation inmore detail.

In another test, the shift of the absorption wavelength due to theDoppler effect is determined on the basis of the change of thewavelength of the light beam necessary for exciting the fluorescence. Inthis test, the use of a second laser beam is not essential. This meansthat, in this case, a particular mirror arrangement providing a secondlaser beam can be omitted. If the single used beam is directed parallelto the motion direction of the particle current, first, an absorption isobtained wherefrom the density of the particle current can bedetermined. Then further measuring of the wavelength shift of the lightbeam, caused by the Doppler effect, relative to the fluorescenceexcitation in the gas at rest, necessary for exciting the fluorescenceof the particles, makes it possible to determine the velocity of theparticles. Therefrom, the rate of particle flow is obtained as theproduct of density times velocity.

The just mentioned test may also be carried out so that the laser beamis adjusted to a definite wavelength at which the particles areabsorbing and the change of the intensity of emission of the fluorescentradiation resulting from the shift of the maximum of absorption in thepresence of a Doppler effect is measured. For example, a current of A1particles produced by cathode sputtering has been excited to fuorescenceby means of a beam having a wavelength of 394.401 nm and, from theabsorption, the density of the particle current has been determined. Inthis case, the beam has been directed against the direction of theparticle current so that, because of the Doppler effect, the wavelengthnecessary for exciting a fluorescence was longer than the wavelength forthe excitation of a gas at rest. The measured value was λ = 6.1×10⁻³ nm.The velocity of the particle current computed therefrom is 4.6×10⁵cm/sec, and the rate of particle flow is

    1.81 ×10.sup.17 cm.sup.-2 sec.sup.- 1.

It should be noted further that, for carrying out the inventive method,it is advantageous to use lasers furnishing an emission line which canbe tuned to an absorption line of the particles to be measured and has ahalf width which is substantially smaller than the half width of therespective line enlarged by the Doppler effect. With known types oflasers, this is easily obtainable. In the shown example, the tuning isobtained by means of grating G the inclination of which relative to thelaser beam is adjusted by an electromagnetic control device EM.

While specific embodiments of the invention have been shown anddescribed in detail to illustrate the application of the principles ofthe invention, it will be understood that the invention may be embodiedotherwise without departing from such principles.

What is claimed is:
 1. A method of determining the rate of flow ofparticles in a vacuum deposition device comprising the steps ofestablishing a current of particles in the device, directing a firstlaser beam through the current, measuring the attenuation of the firstlaser beam due to absorption by the particle current and therefromdetermining the vapor density within the current, determining the firstlaser beam absorption wavelength by observing the beam after it haspassed through the current, passing a second laser beam through thecurrent in a direction different from the first laser beam direction anddetermining the second laser beam absorption wavelength after it haspassed through the current, deriving the difference of absorptionwavelengths of the two laser beams as an indication of the velocity ofthe current, and determining the rate of flow of the particles by usingthe relationship that the rate of flow of the particles is equal to thesaid vapor density times the said velocity.
 2. A method according toclaim 1, wherein a second laser beam, having the same wavelength as thefirst laser beam, is directed through the particles in a respectivedirection transverse to the respective direction of the first laserbeam, and wherein the second laser beam is detected after it has passedthrough the particles and the difference between the respectivetransmissions of the two detected laser beams is measured to show theunequally strong absorption by the particles, the rate of flow of theparticles being determined in accordance with the formula

    R = c.sub.1 (log T.sub.1) c.sub.2 (T.sub.2 - T.sub.1 .sup.c3)

wherein T₁ and T₂ are the transmissions of the particle current for thelaser beams in their respective directions and c₁, c₂ and c₃ areconstants depending on the geometric arrangement and on the coefficientof absorption of the measured particles at the wavelength used formeasuring.
 3. A method according to claim 2, characterized in that,first, T₁ is measured at a predetermined point of the spectrumcorresponding to a maximum of absorption of the kind of particles to bemeasured and, thereupon, the wavelength of the laser beam is changed byΔ λ so as to obtain a minimum for T₂, wherefrom the rate of flow of theparticles results in accordance with the formula

    R =  c.sub.1 (log T.sub.1) c.sub.4 Δ λ

where c₄ is a constant depending on the geometric arrangement and on thecoefficient of absorption of the particles to be measured.
 4. A methodaccording to claim 3, characterized in that a laser radiation is usedhaving a wavelength such that a resonance fluorescence is excited in theparticle current to be measured, and that the shift of the absorptionwavelength, due to the Doppler effect, is determined on the basis of thewavelength shift necessary for exciting the fluorescence.
 5. A methodaccording to claim 4, characterized in that the shift of the absorptionwavelength, in the presence of a Doppler effect, is determined on thebasis of the variation of the intensity of emission of the fluorescenceradiation resulting from this shift.